An ultraviolet light (UV) filter element is employed as a sharp cut filter to transmit visible light while blocking UV light in either an optical device or to protect an underlying element. Such applications include color correction of an image during optical recording (e.g., in either silver halide or digital photography), color correction in a display device (e.g., backlit or reflective liquid crystal displays or emissive displays, OLED), or maintenance of appearance and function of a multicomponent device. Protective functions of such an UV filter element may be provided in the form of protective polymeric overcoats, interlayers, or cast or molded films or other articles used to stabilize accompanying materials against the degradation of optical and mechanical properties induced by UV light exposure (e.g., degradation of liquid crystal components in an LCD display).
An UV filter element is required to thoroughly transmit light of longer than a specified wavelength, while thoroughly blocking light shorter than that wavelength. Optimum performance is provided by a very sharp transition between these wavelengths (i.e., sharp cut). In addition, light transmitted in the visible region should be of neutral hue as perceived by the human eye (e.g., as measured according to the CIE, International Commission on Illumination). The functional performance of the UV filter element should remain as constant as possible during the normal lifetime of the application and under the standard environment of use. That is, in addition to stable mechanical properties, the UV filter element should not significantly lose its ability to transmit visible light (discolor) or block UV light (fade) under normal use in its intended application.
Various classes of UV absorbing compounds are known, and many have been suggested for use in UV filter elements. U.S. Pat. No. 4,043,639, e.g., suggests the use of salicyclic acid esters, hydroxybenzophenone, benzotriazole and their derivatives as UV light-absorbing agents in cellulose triacetate films bonded to a polarizing film for the protection of the liquid crystal components from degradation in a LCD display. The disclosure includes the description of improved resistance to high temperature and humidity of the device. Many classes of UV absorbing compounds previously disclosed for use in polymeric UV filter elements, however, have various problems associated therewith, especially with regard to providing a desired sharp cut at approximately 390 to 400 nm while also providing complete protection throughout the UV range. U.S. Pat. No. 5,806,834 discloses the use of aminobutadiene derivatives to provide a sharp cut UV filter in the 400 nm region with reduced yellow discoloration of the filter as results from high loading levels of benzotriazole or benzophenone derivatives. The formulations described include a second UV absorbing compound added to the polymer in order to absorb light of shorter wavelengths that cannot be absorbed by the primary aminobutadiene compound. Compounds such as benzotriazole, salicylate, benzophenone and their derivatives are described as fulfilling this function. Aminobutadiene compounds, however suffer from thermal instability.
Additionally, use of various compounds as suggested in the prior art still may not provide as sharp a cut-off of absorption between the ultraviolet and visible spectrum as may be desired. It would accordingly be desirable to provide an UV filter element that provides a very sharp cut in transmission between the UV and visible light spectral regions, while also providing effective UV absorption throughout the UV range, as well as good thermal stability.